A ways back I did some analysis on the shield and it's application in D&Dish gaming. Lot's of number crunching most of it showing how 1 pt to AC is sorta wimpy and sells the shield short as they are darned useful in actual combat. Here's another idea, not brand new but not too tricky.
Shield rule XXII
1. Shields have HP scores (suggesting 6 for wooden, 9 for iron-bound, 12 for all metal)
2. Modify AC as per the rules you are playing with.
3. The damage on any even scoring damage blow suffered by a shield-bearer, when the shield counts, is divided between the shield and the shield-bearer. Shields are ruined when they run out of HP.
(optional a). Excess damage beyond what the shield can take is ignored
(optional b). Excess daamge beyond what the shiled can take is suffered by the shield bearer.
(optional c). Missile weapons only inflict 1 pt of damage to a shield, with the exception of throwing axes and darned big missiles.
(optional d). Firearms punch through shields, the shield only takes 1 pt of damage and the remainder suffered by the shield-bearer (unless you have say bullet-proof shields).
Advantage... shields are darned handy, certainly so at low level or when fighting mannish foes.
Disadvantage- there's another score to keep track of (good luck enofrcing that) and math.
What about magic shields? Well... uh yeah, what about them? I suppose there are 3 general options:
I. Magic shields aren't damaged by blow from normal weapons.
II. Magic shields arent damaged by normal weapons scoring damage less then the shield HP total.
III Magic shields have a lot more HP (10 more per plus)
No numerical analysis this time (I'll have to work that up).